Arrangement for a continuous diffuser for washing pulp

ABSTRACT

Arrangement for a continuous diffuser for washing pulp, comprising a number of hydraulic cylinders distributed in a ring, arranged so as to repeatedly raise and then lower a screen assembly which is included in the diffuser. The characterizing feature of the invention is that groups of two or more hydraulic cylinders which lie adjacent to each other in the ring are coupled in parallel with each other, but in series with the next group of adjacent hydraulic cylinders which are also coupled in parallel, so that each group of parallel-coupled hydraulic cylinders is coupled in series with the next group of parallel-coupled hydraulic cylinders.

TECHNICAL FIELD

The invention relates to an arrangement for a continuous diffuser forwashing pulp, comprising a number of hydraulic cylinders distributed ina ring, arranged so as to repeatedly raise and then lower a screenassembly which is included in the diffuser.

PRIOR ART

The washing of paper pulp for the purpose of removing undissolvedresidues of wood and chemicals, for example after cooking and bleaching,can take place in a so-called continuous diffuser. The washing is inthis case effected by means of the pulp being pumped into the bottom ofa tank and moving upwards in the latter. The liquid with dissolved woodresidues and chemicals which surrounds the pulp runs into, and ispressed out in, a screen assembly which is situated in the diffuser andwhich moves upwards at approximately the same speed as the pulp, whileat the same time washing liquid is conveyed to the pulp in connectionwith the screen assembly. When the screen assembly has reached itsuppermost position, it is drawn quickly down to its lowest position,after which the procedure is repeated.

The screen assembly consists of screen elements made of metal, carriedby radial support arms, arranged in concentric rings with diameters of,at present, up to about 9 m. On account of the size, weight andstructure of the screen assembly, it is sensitive to uneven loads whichcan lead to damage and/or operational shutdown. The movement of thescreen assembly is effected with the aid of a plurality of double-actinghydraulic cylinders which are distributed in a ring and whose upperpiston rods are connected to the radial supporting arms of the screenassembly.

In order to obtain a simultaneous parallel movement of the hydrauliccylinders, these are coupled in series, in one or more circuits. Thealternative involving a purely parallel coupling of the hydrauliccylinders means that a drop in pressure in the conduits of the hydraulicsystem has a greater effect, necessitates simultaneous valve movementsin the hydraulic system, and therefore places greater demands onmonitoring equipment. Depending on the size of the screen assembly, thenumber of hydraulic cylinders used at present is either three, four orsix. If there are three hydraulic cylinders in the diffuser, these arecoupled in series; if there are four, they are coupled 2 by 2, i.e. twocircuits of two series-coupled hydraulic cylinders. If there are six,they are coupled 2 by 3, i.e. two circuits of three series-coupledhydraulic cylinders.

In a series-coupled circuit of hydraulic cylinders, the compressibilityof the hydraulic medium leads to the hydraulic cylinders situated in themiddle, if there are more that two of them, working with less power thanthe first and last, on account of the compressibility of the hydraulicmedium. In addition, leakage in a hydraulic cylinder can result in itsnot participating in the work at all, or only to a very small degree.This leads to overloading of the radial supporting arms which lienearest the region to which the leaking hydraulic cylinder is connected.The compressibility of the hydraulic medium and possible leakage in thehydraulic cylinders thus leads to stresses in the screen assembly andits radial supporting arms during the upward and downward movements ofthe said screen assembly, and can additionally lead to damage or to themovement of the screen assembly being arrested, with operationalshutdown as a consequence.

The downward movement of the screen assembly is rapid, in order amongother reasons to clear the screens of pulp by means of so-calledback-flushing. In order to slow down this movement before the hydraulicpistons have reached their lowest position, the movement of thehydraulic pistons is slowed down by means of the lower part of thehydraulic pistons and the hydraulic cylinders being designed as brakechambers. The brake chambers have very small mechanical tolerances, andit is for this reason, and on account of temperature variations and thecompressibility of the hydraulic medium, that the slowing down candiffer between the brake chambers of different hydraulic cylinders,which fact also leads to stresses in the screen assembly and itssupporting arms.

SUMMARY OF THE INVENTION

One object of the invention is to eliminate or to substantially reducethe abovementioned problems. This can be achieved by virtue of the factthat two hydraulic cylinders which lie adjacent to each other in thering are coupled in parallel with each other, but in series with thenext pair of adjacent hydraulic cylinders which are also coupled inparallel, so that each pair of parallel-coupled hydraulic cylinders iscoupled in series with the next pair of parallel-coupled hydrauliccylinders. The construction is such that the two parallel-coupledhydraulic cylinders in each pair are the equivalent of a singlehydraulic cylinder working midway between the two actually existingcylinders. A construction with six hydraulic cylinders thus comes to actas a hypothetical three-cylinder construction with reduced sensitivityto leakage in an individual cylinder in the pair. This results in a moreuniform operation between the hydraulic cylinders, with less sensitivityto leakage and to the compressibility of the hydraulic medium, withreduced stresses on the screen assembly and its radial supporting armsas a consequence.

The number of radial supporting arms in the screen assembly varies inaccordance with the size of the continuous diffuser. The larger thediffuser, the more supporting arms are needed for the mechanicalstability of the screen assembly and for managing the movement of liquidto and from the screen assembly. The number of radial supporting arms inthe present invention is an even one, specifically four or six, in orderamong other reasons to gain full advantage of the abovementionedparallel coupling in accordance with the present invention.

The hydraulic cylinders in the present invention are double-acting, i.e.they manage both the upward and the downward movement of the screenassembly. The speed of the upward movement of the hydraulic pistons issuch that the screen assembly is moved upwards slightly faster than theupward movement of the pulp in the diffuser, i.e. it takes about oneminute from its lowest to its uppermost position. The speed of thedownward movement of the hydraulic pistons is such that it takes lessthan one second from its uppermost position to its lowest position.

Another object of the invention is to equalize the slowing down of thehydraulic pistons at the end of their rapid downward movement. This isachieved by coupling together the brake chambers formed at the bottom ofthe hydraulic cylinders between the two hydraulic cylinders in each pairof parallel-coupled, adjacent hydraulic cylinders in accordance with theabove, which results in a more even slowing down between neighbouringhydraulic cylinders and therefore less stress on the screen assembly andits supporting arms.

Further characteristics, aspects and advantages of the invention areevident from the following description of an embodiment and from theattached patent claims.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows the diffuser in a cutaway view.

FIG. 2 shows the coupling of the hydraulic cylinders in a system withsix hydraulic cylinders according to a previous design.

FIG. 3 shows the coupling of the hydraulic cylinders in a system withsix hydraulic cylinders according to the invention.

FIG. 4 shows diagrammatically the paired coupling of the brake chambersof the hydraulic cylinders.

DETAILED DESCRIPTION

The production of paper pulp takes place in an aqueous phase. Inconstituent processes during production, parts of the wood are dissolvedunder the action of chemicals, inter alia. These wood residues andchemicals, and conversion products thereof, are left in the aqueousphase together with the pulp. In order to remove these, or, as we say,to wash the pulp, the impure aqueous phase which surrounds the pulp isreplaced with a purer aqueous phase by means of the impure aqueous phasebeing allowed to run off or be pressed out, while at the same time apurer aqueous phase is supplied. This is carried out in, for example,presses, on rotating filters, or in a so-called continuous diffuser. Thepresent invention relates to an arrangement for a continuous diffuserfor washing pulp, comprising a number of hydraulic cylinders distributedin a ring, and arranged so as to repeatedly raise and then lower ascreen assembly which is included in the diffuser.

FIG. 1 shows a continuous diffuser 1 in which the pulp is pumped intothe bottom and moves upwards in the latter. The diffuser contains ascreen assembly 2 which is movable in the vertical direction and whichconsists of a number of concentric screen rings 3 joined together byradial supporting arms 4. The piston rods 5 on the hydraulic cylinders 6are connected to the supporting arms 4. Washing liquid is suppliedthrough the washing-liquid inlets 7 and is spread out, by means ofrotating devices 8, between the concentric screens 3 in the screenassembly 2. Impure liquid is drawn off from the screen assembly 2through the supporting arms 4 and out through the outlet nozzles 9. Thepurified pulp is removed at the top of the diffuser by means of the pulpbeing fed by a rotating scraper 10, driven by the motor 11, into achannel 12, after which the pulp runs down through a shaft 13 to astorage container 14 or to a pump.

The liquid with dissolved wood residues and chemicals which surroundsthe pulp runs into, and is pressed out in, a screen assembly 2 which issituated inside the diffuser 1. From its lowest position the screenassembly 2 moves upwards, at a slightly greater speed than that of thepulp, with the aid of the hydraulic cylinders 6 which are connected viatheir piston rods 5 to the supporting arms 4, while at the same timewashing water is supplied to the pulp through devices 8 between theconcentric screens 3 in the screen assembly 2, and at the same time asimpure liquid is drawn off through the outlet nozzles 9. When the screenassembly 2 has reached its uppermost position, it is drawn quickly downto its lowest position by the hydraulic cylinders 6, at the same time asliquid is back-flushed through the screens. The movement is quick, andback-flushing is carried out in order to clear the screens of fibres andother solid impurities. The procedure is then repeated by means of theupward movement being begun again. The purified pulp is removed at thetop of the diffuser by means of the pulp being fed by a rotating scraper10 into a channel 12, after which the pulp runs down into a storagecontainer 14, or to a pump.

In order to ensure that the hydraulic cylinders work simultaneously anduniformly, the hydraulic cylinders are coupled in series, two series ifthey are four or more in number. In theory, this means that each amountof liquid, in each of the series, which is forced down by the piston inthe first hydraulic cylinder in the series, acts at the same time, andwith the same amount, on the piston in the next hydraulic cylinder inthe series, etc. With the exception of the compressibility of the oil,this guarantees parallel movement of the cylinders and consequently ofthe screen assembly, as long as none of the cylinders leaks. In orderfurther to guarantee there is no inclined movement, the time differencebetween the times when the pistons reach their end positions ismeasured, inter alia, and these times may not exceed certain limitvalues, in which case the movement is arrested.

FIG. 2 shows an installation with six hydraulic cylinders A, B, C, D, Eand F, fed from a hydraulic unit 19. When six hydraulic cylinders areused, these are coupled 2 by 3, i.e. in two series-coupled circuits withthree hydraulic cylinders in each circuit, as per FIG. 2, in order tocompensate for any differences in their manner of operation. As is clearfrom FIG. 2, the hydraulic cylinders A, C and E are coupled in series inone circuit, and the hydraulic cylinders B, D and F in series in thenext circuit. The coupling is additionally configured such that themiddle hydraulic cylinders C and D in each circuit, which cylinders workless efficiently than the others on account of the compressibility ofthe hydraulic medium, are placed straight opposite each other.

The downward movement of the screen assembly is rapid. In order to slowdown this movement before the hydraulic pistons have reached theirlowest position, the movement of the hydraulic pistons is slowed down bymeans of the lower part of the hydraulic pistons and the hydrauliccylinders being designed such that the flow of the hydraulic medium isreduced, by means of a throttle, before the hydraulic pistons havereached their lowest position. That part of the hydraulic cylinder whichhas been designed for throttling the flow of hydraulic medium, for thepurpose of slowing down the rapid downward movement, is hereinafterreferred to as the brake chamber.

The shortcomings of the known technique are therefore that thecompressibility of the hydraulic medium leads to a situation where, in aseries-coupled circuit of at least three hydraulic cylinders, thosehydraulic cylinders situated in the middle work with less force than thefirst and last ones on account of the compressibility of the hydraulicmedium. In addition, leakage in a hydraulic cylinder can result in itsnot participating at all in the work, or participating only veryslightly. This leads to overloading of the radial supporting arms whichlie nearest the region on which the leaking hydraulic cylinder acts. Thecompressibility of the hydraulic medium and possible leakage in thehydraulic cylinders thus leads to stresses in the screen assembly andits radial supporting arms during the upward and downward movementsthereof, and can additionally lead to damage or to the movement of thescreen assembly being arrested, with operational shutdown as aconsequence.

The brake chambers additionally have small mechanical tolerances, and itis for this reason, and on account of temperature variations and thecompressibility of the hydraulic medium, that the slowing down candiffer between the brake chambers of different hydraulic cylinders,which fact also leads to stresses in the screen assembly and itssupporting arms.

The couplings of the hydraulic cylinders which are described beloweliminate or substantially reduce the abovementioned problems.

DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 3 shows the preferred embodiment according to the invention,consisting of six double-acting hydraulic cylinders A', B', C', D', E'and F' which are coupled in pairs. The upper ends of the piston rods 36are connected to the ends of the radial supporting arms of the screenassembly, directly or via chambers for conveying liquid to or from thescreen assembly. The hydraulic cylinders A', B', C', D', E' and F' areconnected to a hydraulic unit 19' via two conduits 35 and 20. Dependingon which stage of the work cycle the installation is in, the conduits 35and 20 are alternately a delivery conduit and a return conduit.

The conduit 20 is connected at one end to a hydraulic unit 19' anddivides at its other end into two conduits 21 and 22, the conduit 21connecting to the upper side of the hydraulic piston of the hydrauliccylinder E', and the conduit 22 connecting to the upper side of thehydraulic piston of the hydraulic cylinder D'. The conduit 23 connectsto the underside of the hydraulic piston of the hydraulic cylinder E'and joins with the conduit 24 which is connected to the underside of thehydraulic piston of the hydraulic cylinder D'. The conduits 23 and 24join to form the conduit 25. The conduit 25 divides at its other endinto two conduits 26 and 27, the conduit 26 connecting to the upper sideof the hydraulic piston of the hydraulic cylinder A', and the conduit 27connecting to the upper side of the hydraulic piston of the hydrauliccylinder B'. The conduit 28 connects to the underside of the hydraulicpiston of the hydraulic cylinder A' and joins with the conduit 29 whichis connected to the underside of the hydraulic piston of the hydrauliccylinder B'. The conduits 28 and 29 join to form the conduit 30. Theconduit 30 divides at its other end into two conduits 31 and 32, theconduit 31 connecting to the upper side of the hydraulic piston of thehydraulic cylinder C', and the conduit 32 connecting to the upper sideof the hydraulic piston of the hydraulic cylinder F'. The conduit 34connects to the underside of the hydraulic piston of the hydrauliccylinder C' and joins with the conduit 33 which is connected to theunderside of the hydraulic piston of the hydraulic cylinder F'. Theconduits 34 and 33 join to form the conduit 35, which connects to thehydraulic unit 19'.

During the upward movement of the screen assembly, in FIG. 3 the conduit35 is the delivery conduit, and the pressure which the hydraulic unit19' supplies is conveyed onwards to the hydraulic cylinders F' and C' byvirtue of the fact that the delivery conduit 35 divides into the twoconduits 33 and 34, the conduit 33 leading to the underside of thehydraulic piston in the hydraulic cylinder F', and the conduit 34leading to the underside of the hydraulic piston in the hydrauliccylinder C'. The hydraulic pistons in the hydraulic cylinders F' and C'thus move upwards at the same time as the hydraulic oil at the top sideof the hydraulic pistons in the hydraulic cylinders F' and C' is pressedout through respective conduits 32 and 31. These conduits 32 and 31 arejoined together to form the conduit 30, which conduit 30, during theupward movement of the screen assembly, is the delivery conduit to thehydraulic cylinders B' and A'.

The conduit 30 divides into the two conduits 29 and 28 which aredelivery conduits to the hydraulic cylinders B' and A', the conduit 29leading to the underside of the hydraulic piston in the hydrauliccylinder B', and the conduit 28 leading to the underside of thehydraulic piston in the hydraulic cylinder A'. The hydraulic pistons inthe hydraulic cylinders B' and A' thus move upwards by means of thehydraulic oil which is pressed out from the hydraulic cylinders F' andC', while at the same time the hydraulic oil at the top side of thehydraulic pistons in the hydraulic cylinders B' and A' is pressed outthrough respective conduits 27 and 26. These conduits 27 and 26 arejoined together to form the conduit 25, which conduit 25, during theupward movement of the screen assembly, is the delivery conduit to thehydraulic cylinders D' and E'.

The conduit 25 divides into the two conduits 24 and 23 which aredelivery conduits to the hydraulic cylinders D' and E', the conduit 24leading to the underside of the hydraulic piston in the hydrauliccylinder D', and the conduit 23 leading to the underside of thehydraulic piston in the hydraulic cylinder E'. The hydraulic pistons inthe hydraulic cylinders D' and E' thus move upwards by means of thehydraulic oil which is pressed out from the hydraulic cylinders B' andA', while at the same time the hydraulic oil at the top side of thehydraulic pistons in the said hydraulic cylinders D' and E' is pressedout through respective conduits 22 and 21. These conduits 22 and 21 arejoined together to form the conduit 20, which conduit 20, during theupward movement of the screen assembly, is the return conduit to thehydraulic unit 19'.

During the downward movement of the screen assembly, in FIG. 3 theconduit 20 is the delivery conduit, and the pressure which the hydraulicunit 19' supplies is conveyed onwards to the hydraulic cylinders E' andD' by virtue of the fact that the delivery conduit 20 divides into thetwo conduits 21 and 22, the conduit 21 leading to the top side of thehydraulic piston in the hydraulic cylinder E', and the conduit 22leading to the top side of the hydraulic piston in the hydrauliccylinder D'. The hydraulic pistons in the hydraulic cylinders E' and D'thus move downwards at the same time as the hydraulic oil at theunderside of the hydraulic pistons in the hydraulic cylinders E' and D'is pressed out through respective conduits 23 and 24. These conduits 23and 24 are joined together to form the conduit 25, which conduit 25,during the downward movement of the screen assembly, is the deliveryconduit to the hydraulic cylinders A' and B'.

The conduit 25 divides into the two conduits 26 and 27, the conduit 26leading to the top side of the hydraulic piston in the hydrauliccylinder A', and the conduit 27 leading to the top side of the hydraulicpiston in the hydraulic cylinder B'. The hydraulic pistons in thehydraulic cylinders A' and B' thus move downwards by means of thehydraulic oil which is pressed out from the hydraulic cylinders E' andD', while at the same time the hydraulic oil at the underside of thehydraulic pistons in the hydraulic cylinders A' and B' is pressed outthrough respective conduits 28 and 29. These conduits 28 and 29 arejoined together to form the conduit 30, which conduit 30, during thedownward movement of the screen assembly, is the delivery conduit to thehydraulic cylinders C' and F'.

The conduit 30 divides into the two conduits 31 and 32, the conduit 31leading to the top side of the hydraulic piston in the hydrauliccylinder C', and the conduit 32 leading to the top side of the hydraulicpiston in the hydraulic cylinder F'. The hydraulic pistons in thehydraulic cylinders C' and F' thus move downwards by means of thehydraulic oil which is pressed out from the hydraulic cylinders A' andB', while at the same time the hydraulic oil at the underside of thehydraulic pistons in the hydraulic cylinders C' and F' is pressed outthrough respective conduits 34 and 33. These conduits 34 and 33 arejoined together to form the conduit 35, which conduit 35, during thedownward movement of the screen assembly, is the return conduit to thehydraulic unit 19'.

In the coupling arrangement described above, each pair of hydrauliccylinders A'+B', C'+F', and E'+D' comes to work with the same pressureat the inlet side and the same pressure at the outlet side, which meansthat each pair comes to work with the same power and with a mean valueof both the hydraulic cylinders' efficiency, which also compensates forany differences between the pairs, which fact in turn makes possible theeffects which have been discussed in the present account of theinvention.

FIG. 4 shows the coupling of the brake chambers according to the presentinvention. The brake chambers at the bottom of each hydraulic cylinderare coupled in pairs in the manner which is shown by the broken lines inFIG. 4, so that the brake chamber in the hydraulic cylinder E' is pairedwith the brake chamber in the hydraulic cylinder D' via the conduit 40,the brake chamber in the hydraulic cylinder A' is paired with the brakechamber in the hydraulic cylinder B' via the conduit 41, and the brakechamber in the hydraulic cylinder C' is paired with the hydrauliccylinder F' via the conduit 42. This paired coupling between the brakechambers of neighbouring hydraulic cylinders means that the pressures inthe brake chambers are equal, which means on the one hand that theslowing down is equal in these two cylinders, and also results in acompensation of the slowing down between the different pairs of brakechambers and, therefore, a more even slowing down of the whole screenassembly, with less mechanical stressing as a consequence.

The foregoing is a complete description of the present invention.Changes and modifications by persons skilled in the art arecontemplated.

What is claimed is:
 1. Arrangement for a continuous diffuser for washingpulp, comprising a number of hydraulic cylinders distributed in a ring,arranged so as to repeatedly raise and then lower a screen assemblywhich is included in the diffuser, characterized in that groups of twoor more hydraulic cylinders which lie adjacent to each other in the ringare coupled in parallel with each other, but in series with the nextgroup of adjacent hydraulic cylinders which are also coupled inparallel, so that each group of parallel-coupled hydraulic cylinders iscoupled in series with the next group of parallel-coupled hydrauliccylinders.
 2. Arrangement according to claim 1, characterized in thatthe number of hydraulic cylinders in the ring is an even one. 3.Arrangement according to claim 2, characterized in that the number ofhydraulic cylinders is 4 or
 6. 4. Arrangement according to claim 1characterized in that the hydraulic cylinders are double acting. 5.Arrangement according to claim 2 characterized in that the hydrauliccylinders are double acting.
 6. Arrangement according to claim 3characterized in that the hydraulic cylinders are double acting. 7.Arrangement according to claim 1 characterized in that the speed of theupward movement of the screen assembly takes about one minute from thelowest position to the uppermost position.
 8. Arrangement according toclaim 2 characterized in that the speed of the upward movement of thescreen assembly takes about one minute from the lowest position to theuppermost position.
 9. Arrangement according to claim 3 characterized inthat the speed of the upward movement of the screen assembly takes aboutone minute from the lowest position to the uppermost position. 10.Arrangement according to claim 4 characterized in that the speed of theupward movement of the screen assembly takes about one minute from thelowest position to the uppermost position.
 11. Arrangement according toclaim 1 characterized in that the speed of the downward movement of thescreen assembly takes less than about one second from the uppermostposition to the lowest position.
 12. Arrangement according to claim 2characterized in that the speed of the downward movement of the screenassembly takes less than about one second from the uppermost position tothe lowest position.
 13. Arrangement according to claim 3 characterizedin that the speed of the downward movement of the screen assembly takesless than about one second from the uppermost position to the lowestposition.
 14. Arrangement according to claim 4 characterized in that thespeed of the downward movement of the screen assembly takes less thanabout one second from the uppermost position to the lowest position. 15.Arrangement according to claim 7 characterized in that the speed of thedownward movement of the screen assembly takes less than about onesecond from the uppermost position to the lowest position. 16.Arrangement according to claim 1 characterized in that the screenassembly includes radial supporting arms which are connected to thehydraulic cylinders.
 17. Arrangement according to claim 2 characterizedin that the screen assembly includes radial supporting arms which areconnected to the hydraulic cylinders.
 18. Arrangement according to claim3 characterized in that the screen assembly includes radial supportingarms which are connected to the hydraulic cylinders.
 19. Arrangementaccording to claim 4 characterized in that the screen assembly includesradial supporting arms which are connected to the hydraulic cylinders.20. Arrangement according to claim 7 characterized in that the screenassembly includes radial supporting arms which are connected to thehydraulic cylinders.
 21. Arrangement according to claim 11 characterizedin that the screen assembly includes radial supporting arms which areconnected to the hydraulic cylinders.
 22. Arrangement according to claim1 including means for braking the lowering of the screen assembly at theend of the of the lowering of said screen assembly.
 23. Arrangementaccording to claim 2 including means for braking the lowering of thescreen assembly at the end of the of the lowering of said screenassembly.
 24. Arrangement according to claim 3 including means forbraking the lowering of the screen assembly at the end of the of thelowering of said screen assembly.
 25. Arrangement according to claim 4including means for braking the lowering of the screen assembly at theend of the of the lowering of said screen assembly.
 26. Arrangementaccording to claim 7 including means for braking the lowering of thescreen assembly at the end of the of the lowering of said screenassembly.
 27. Arrangement according to claim 11 including means forbraking the lowering of the screen assembly at the end of the of thelowering of said screen assembly.
 28. Arrangement according to claim 16including means for braking the lowering of the screen assembly at theend of the of the lowering of said screen assembly.
 29. Arrangementaccording to claim 22 characterized in that said means for brakingcomprises at least one brake chamber.
 30. Arrangement according to claim22 characterized in that said means for braking includes at least onebrake chamber associated with each hydraulic cylinder, said brakechambers coupled together in pairs corresponding to the pairs ofparallel-coupled hydraulic cylinders.